<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>三角形</title>

    <style type="text/css">
        .box{
            width: 0px;
            height: 0px;
            /* background-color: aquamarine; */
            /* border-top: 50px solid transparent;
            border-right: 50px solid transparent;
            border-left: 50px solid transparent;
            */
            border: 50px solid transparent;
            border-bottom: 50px solid red; 
        }

    </style>
</head>
<body>

    <div class="box"></div>
    
</body>
</html>